In a known X-ray photography system, an object, especially the interior of the object, is observed by measuring the radiation transmission distribution of the object. X-rays typically are employed as the radiation. In a system of this kind, a method that is becoming the norm involves acquiring a radiation distribution using a large-format image sensor that employs a solid-state image sensing device referred to as a flat-panel X-ray sensor.
A solid-state image sensing device is characterized by having a multiplicity of pixels on a plane and sampling a planar energy distribution spatially in direct fashion to so that the distribution can be converted to an electric signal. While this feature is an advantage of such a device, in a certain sense it is a disadvantage as well. Specifically, since the spatial distribution is sampled directly, a spatially continuous signal state in which sampling can be performed freely does not exist.
For example, when sampling something that has been converted to a video signal by a lens scanner using a conventional photo-stimulable phosphor, the fact that a spatially continuous signal exists on the photo-stimulable phosphor as a latent image means that sampling can be carried out again at any sampling pitch after X-ray photography has been performed. Similarly, in a case where an image that has been visualized on a silver-halid film is read and converted to a digital signal, there is substantial latitude in terms of the sampling pitch.
On the other hand, with a flat-panel X-ray sensor in which a radiation-dose spatial distribution that exists momentarily owing to exposure to radiation is spatially sampled directly by a plurality of pixel sensors arrayed side by side in actual space, the sampling interval is dependent upon the a pre-designed sampling pitch. If it is desired to broaden the sampling pitch, an expedient of the kind described in the specification of Japanese Patent Application Laid-Open No. 10-285466, Japanese Patent No. 2000-504516 or U.S. Pat. No. 3,031,606 is employed. Specifically, a sensor having a sampling pitch finer than that usually required is designed, pixel information signals from mutually adjacent pixels on the sensor are added as electric signals and, under ordinary circumstances, the sensor is essentially handled as one having large pixels and a large pixel pitch. When finer information is required, the adding of signals is halted and information is extracted on a pixel-by-pixel basis.
However, once mutually adjacent pixels have been added and image information captured at a substantially large pixel pitch, it is impossible to generate image information from this image information at a sampling pitch finer than that of this image information. When finer information is required, therefore, it is necessary to perform photography again. In order to avoid this, image information obtained as a digital signal by sampling at a fine pitch from the start is saved, pixels are added in the form of a digital signal and image information that is essentially coarse is obtained. Since the fine image information also is saved in this case, the fine image information can be called.
With this approach, however, the amount of image information held is greater than necessary and there is the danger that the sampling rate of the image information will rise more than necessary. As a consequence, there are instances where cost rises so high as to make the above arrangement substantially impractical.